The present invention generally relates to apparatus and methods for limiting damage to generators, other rotating machines and associated driving devices and prime movers such as gearboxes and engines and, more particularly, to apparatus and methods of limiting damage initiated by a bending failure of a stub shaft affixed to the main rotor shaft.
Rotating machines, such as aircraft generators, often have a stub shaft which interfaces between the main rotor shaft of the generator and the prime mover gearbox. The stub shafts are coupled to the main engine or to auxiliary power units through the gear box. If the main rotor shaft is or becomes misaligned, for example from a ball bearing failure, a bending type failure of the stub shaft may occur. This means that the stub shaft breaks apart but not smoothly at a line perpendicular to a length of the stub shaft but rather the break may be along an incline such as a 45° incline.
When the bending failure of the stub shaft develops and the stub shaft breaks at an inclined surface, since the two parts of the broken stub shaft attempt to turn at different speeds, an axial load is imparted toward the main rotor shaft and gearbox shaft. Typically the stub shaft has a flange proximal to the end of the main rotor shaft. The reason for the presence of this flange is to limit movement of the stub shaft. Accordingly if the stub shaft is axially constrained in the gearbox output shaft, the axial load resulting from the initial bending failure is transmitted via the flange to the main rotor shaft from the proximal end of the main rotor shaft throughout the main rotor shaft to its distal end and then to the housing of the rotating machine. The housing of the generator may be vulnerable to breakage since it is often comprised of a first portion and an end bell portion bolted together nearer the distal end of the main rotor shaft. A broken generator housing would cause further damage to the generator potentially resulting in an uncontained failure and significant consequential damage to adjacent equipment.
A second problem from the bending failure of the stub shaft is that the main rotor shaft may keep turning. This might create excessive vibration and consequential damage to adjacent equipment.
As can be seen, there is a need for an improved assembly or system that limits or contains any potential damage to from a bending type failure of the stub shaft.